With respect to the preparation of aromatic polycarboxylic acids, it has hitherto been proposed to synthetize benzoic acid and terephthalic acid respectively from chlorobenzene and 4-chlorobenzoic acid, for example, by the CO-insertion reaction at high temperature and under high pressure in the presence of a carbonylation catalyst such as cobalt carbonyl or by the phase-transfer catalyst method. However, it has not yet been reported that the desired product may be obtained with a high yield. Also it has not yet been reported that, for example, 4-chlorophthalic acid, 5-chlorotrimellitic and the like may be converted to the corresponding polycarboxylic acids such as trimellitic acid, pyromellitic acid and the like by the CO-insertion reaction.
Recently, studies on the photocarbonylation of chlorobenzene in the presence of a catalyst have been reported and moreover a study on the preparation of terephthalic acid by photocarbonylation of 4-chlorobenzoic acid in the presence of a cobalt carbonyl has been reported (Jean-Jacques Brunet, Christian Sidot & Paul Caubere, J. Org. Chem., 1983, 48, 1166-1171).
It was previously known in the art that, in the synthesis of aromatic carboxylic acids, the reaction becomes more difficult and the amount of by-products increases in accordance with the increase of the number of carboxyl groups.
It has now unexpectedly found that, by subjecting a monochloro-polycarboxylic acid to the photocarbonylation reaction in a homogeneous aqueous solution, the reaction proceeds smoothly and the reaction velocity becomes considerably faster than the corresponding velocity of the photocarbonylation reaction of 4-chlorobenzoic acid, and that by-product is scarcely formed.